Antibodies are capable of specifically binding to substances called antigens, and detoxifying and removing antigen-containing factors with the cooperation of other biomolecules or cells. The name “antibody” is particularly based on such a capability of binding to an antigen, and is also referred to as “immunoglobulin” as a chemical name.
Substances (ligands) capable of specifically binding to constant regions (regions other than antigen-binding sites) of antibodies can be used to purify or detect antibodies. Their ability to bind to constant regions enables them to be used for extended applications, and therefore their industrial importance is sufficiently high. As typical examples of ligands capable of specifically binding to constant regions of antibodies, there may be mentioned Protein A and Protein G.
The development of antibody-related industries such as antibody drugs and laboratory diagnosis has increased the demand for more chemically stable ligands. In particular, there is a strong demand for ligands that are highly stable against alkali because alkalis are used in the deactivation of viruses or the like, or in the washing of carriers (e.g. beads or chips) with immobilized ligands. Since polypeptide ligands are generally vulnerable to alkali attack, many studies have been performed to improve their alkali resistance while maintaining their excellent binding specificity (Non Patent Literatures 1 and 2).
Examples of techniques for improving the alkali resistance of a ligand that binds to an antibody include techniques of mutating Gly at position 29 of Protein A to provide alkali resistance (Patent Literatures 1 and 2); techniques of replacing Asn with another amino acid to provide alkali resistance (Patent Literatures 3 and 4); and techniques using C domain of Protein A or a variant thereof (Patent Literatures 5 and 6).